Phenylketonuria (PKU) has played a prominent role in the development of modern genetic medicine. Individuals with this rare inherited disease are unable to metabolize phenylalanine, an essential amino acid found in all dietary proteins. In the absence of treatment, ingested phenylalanine and its metabolites accumulate to toxic levels in blood and other tissues, leading to serious neuropsychological abnormalities, including profound mental retardation. However, as the result of population-wide screening and treatment, most of those born with PKU are now able to complete high school, hold jobs, and raise families. Because of its rarity, few medical graduates will ever see a single case. Yet every physician in the country knows about the condition, which is employed in medical school curricula to illustrate the effectiveness of scientific medicine, and of the benefits inherent in the collaboration of the laboratory and the clinic. But familiarity with PKU extends well beyond clinical medicine. It has come to exemplify the benefits of genetic testing, and because it demonstrates that the expression of a genetic trait may be drastically altered by changes in the environment, has also been frequently invoked in the nature-nurture debate. PKU has thus played a truly singular role in both the history of public-health and human genetics. This policy-oriented history of newborn screening for PKU utilizes published and archival materials, interviews with scientists and others instrumental in the development of screening, and interviews with persons who have PKU to, 1) explore how a disease of only marginal public health significance became the object of an unprecedented system for the mandatory routine testing of every infant born in this country and a powerful symbol in the discourses of medical and behavior genetics, 2) present a balanced account of the strengths and limitations of this first effort to manage a heritable disease based on widespread testing and, 3) illuminate broader socio-cultural trends, particularly in respect to social controls on reproduction. [unreadable] [unreadable]